Conventionally, a navigation apparatus which displays a map on a display and performs route guidance is known. When a user sets up a destination, this navigation apparatus searches for an optimal route from the current position of a vehicle equipped with the navigation apparatus to a destination, and stores this optimal route as a route to be traveled. When the vehicle starts traveling and the navigation apparatus starts route guidance, the navigation apparatus carries out detection of the current position of the vehicle sequentially, displays the current position of the vehicle and the route to be traveled on an on-screen map including the detected current position, and makes guidance. When the vehicle then reaches a vicinity of a branch point of the route to be traveled, the navigation apparatus displays a guide map consisting of a three-dimensional image which is easy for the driver to compare with a forward view which the driver is actually seeing, and also provides guidance for the driver by making a sound. Thus, the navigation apparatus enables the user to easily select the traveling direction at the time when the vehicle is travelling the route to be traveled with the map and the guidance using the guide map.
For such a navigation apparatus, a technology of reproducing a view which is close to an actual view which can be observed from the vehicle on the screen is known. For example, patent reference 1 discloses an image display method of displaying an image, such as a map associated with a time or a map having the sense of a season. In accordance with this image display method, a calendar timer monitors the current date, a multiplication constant generating unit generates multiplication constants CR, CG, and CB for R, G, and B data, respectively, in such a way that the image has a hue according to the current time, the current season, or the like, multipliers for R, G, and B multiply pieces of color information (RGB data) of the map outputted from a color palette by the multiplication constants CR, CG, and CB, respectively, so as to manipulate the pieces of color information of the map image, and the image is then displayed on the basis of the pieces of color information acquired through this manipulation. According to this image display method, the display color of the map screen can be changed in synchronization with either a preset time zone or the current season, an atmosphere of daytime or nighttime can be expressed, and the visibility can be improved.    [Patent reference 1] JP,9-311623,A
A problem with the technology disclosed by above-mentioned patent reference 1 is, however, that because color palettes each having a color tone which is suited to one of various situations are prepared in advance, and, when displaying an image, a color palette which is optimal at that time is selected and the image is displayed, when, for example, the number of situations which are set up in advance is n and the number of maximum colors which can be defined for each color palette is m, the number of colors which can be expressed in one certain situation is restricted to m/n. Another problem is that in the case of displaying an image in a bitmap format, because the maximum number of colors which can be define for each color palette is restricted to 256, the image cannot be displayed realistically. A further problem is that because a color palette having a different color tone is applied to image data except color information, there is a restriction that the image itself cannot be changed according to the situation.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a navigation apparatus which can display a guide map according to the current situation without limit on the number of colors.